Multiphase sampling of modulated light with phase synchronization field

ABSTRACT

A light transmitter transmits multiple light packets, each formatted to include a predetermined phase synchronization field (PSF) and a same message comprising a series of bits. The PSF and each bit are each represented as light that is intensity modulated over a bit period at a corresponding frequency. The light packets are transmitted at different start-times to cause a receiver to sample each packet with a different phase of a fixed, asynchronous sample timeline. The PSF and message are demodulated from each of the sampled light packets. If the demodulated PSF matches the predetermined PSF, then the corresponding demodulated message is declared valid.

BACKGROUND

A light communication system may include a light transmitter to transmit data bits in modulated light packets to a light receiver, such as a camera. The light packets modulated to convey the data bits at a transmit bit rate. A logic level of each transmitted data bit may be represented as light that is intensity modulated to indicate the logic level. The camera samples the received light packets once every camera frame at a frame rate of the camera, to produce light samples at the frame rate. The data bits may then be demodulated based on the light samples.

Ideally, the transmit bit rate and the camera frame rate (or sample rate) are synchronized so as to produce consistent, error free samples that result in correctly demodulated bits in the receiver. In practice, however, the transmit bit rate and the camera frame rate are not often synchronized because the transmitter and the receiver operate based on their respective different clocks that are not synchronous with each other, i.e., their clocks are asynchronous. When the transmit bit rate and the frame rate are not synchronized, then the transmitted data bits slowly “slip” through or “drift” by the camera frames or sample times. Due to this drift, occasionally camera sample times will coincide with and thereby sample edge transitions in the intensity modulated light, which results in an indeterminate sample, i.e., a sample that has an indeterminate level. Disadvantageously, this may result in an erroneous demodulated data bit.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1A is an illustration of an example light array, which may operate in accordance with embodiments described herein.

FIG. 1B is an illustration of another example light array, which may operate in accordance with the embodiments described herein.

FIG. 1C is an illustration of yet another example light array, which may operate in accordance with the embodiments described herein.

FIG. 2 is a diagram of an example light communication system employing spatially-separated beams.

FIG. 3A is a block diagram of an example light communication system and an example light transmitter useful to introduce the principles of frequency shift on-off keying (FSOOK) modulation and detection/demodulation, as it applies to the embodiments described herein.

FIG. 3B is a block diagram of a light receiver from FIG. 3A, according to an embodiment.

FIG. 3C is a block diagram of a light imager including light sample digitizing modules, according to an embodiment.

FIG. 4A is an illustration of an example timing diagram of a frequency shift keying (FSK) waveform corresponding to an FSK signal from FIG. 3A.

FIG. 4B is an illustration of an exemplary light packet definition or light packet protocol for light packets formatted and transmitted by the light transmitter of FIG. 3A.

FIG. 5 is a light amplitude/intensity vs. time diagram helpful in understanding how a light receiver detector/demodulator of FIG. 3B associates light samples with demodulated data bits.

FIG. 6 is a block diagram of an example multi-light transmitter to transmit light packets.

FIG. 7 is a timing diagram of exemplary light packets transmitted by a light transmitter, in accordance with sequential multiphase transmission.

FIG. 8 is a timing diagram of exemplary light packets transmitted by a light transmitter, in accordance with parallel multiphase transmission.

FIG. 9 is an example timing diagram at a light receiver corresponding to the sequential and parallel multiphase transmission embodiments.

FIG. 10 is a diagram of an example of multiphase transmission and reception, in which a light transmitter transmits three multiphase light packets each carrying the same 10-bit data message, with multiphase offsets.

FIG. 11 is an exemplary light packet definition for FSK modulated light packets used in multiphase transmission with phase synchronization field, according to an embodiment.

FIG. 12A is a timing diagram of exemplary transmit light packets transmitted by a light transmitter, in accordance with sequential multiphase transmission with phase synchronization field (PSF).

FIG. 12B is a timing diagram of exemplary transmit light packets transmitted by a light transmitter, such as multi-light transmitter, in accordance with parallel multiphase transmission with PSF (PSF).

FIG. 13 is a flowchart of an example method summarizing multiphase transmission embodiments (without PSF).

FIG. 14 is a flowchart of an example method summarizing multiphase transmission with PSF, according to an embodiment.

FIG. 15 is a block diagram of an example computer processor system configured for multiphase sampling processing.

FIG. 16 is a block diagram of an example system including a system or apparatus to sample and record light beams as a sequence of images, and process the recorded images in accordance with one or more embodiments described herein.

In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears.

DETAILED DESCRIPTION

Described below are embodiments directed to multiphase sampling of light packets to reduce deleterious effects otherwise caused when a light transmitter and a light receiver are not synchronized. Multiphase sampling can be used with an arbitrary light transmitter that transmits light to the light receiver, such as a camera. Each transmitted light packet may be formatted to include sequential fields of modulated light, including an efficient start-frame-delimiter (SFD), followed by a data message, comprising a series of data bit values to be transmitted at a transmit bit rate, e.g., at 15 bits-per-second (bps). The light corresponding to the data message may be modulated in accordance with a modulation technique referred to as frequency shift on-off keying (FSOOK). In FSOOK, each bit level may be represented as light that is intensity modulated (e.g., cyclically keyed between HIGH and LOW intensity levels) over a bit period, at one of multiple frequency shift keying (FSK) frequencies, each indicative of one of multiple bit levels (i.e., logic 0 and logic 1). In other words, each bit level is represented as a corresponding FSK waveform of intensity modulated light.

The light receiver receives the transmitted light packets and samples the FSK waveform at a receive sample rate or frame rate, e.g., 30 frames-per-second (fps), to produce light samples at that rate. The light receiver demodulates the data bits of the received light packets based on the light samples. Demodulation is facilitated by maintaining, as near as possible, a harmonic relationship between the transmit bit rate and the receive sample rate. As mentioned above, multiphase sampling compensates for a lack of synchronization between the receive sample rate and the transmit bit rate (and the FSOOK frequencies associated with the bit levels), i.e., when a strict harmonic relationship between the two rates is not maintained.

The ensuing description is divided into the following sections:

Light Arrays Light Beam Diagram Light Communication System Using FSOOK Light Transmitter Light Receiver Multi-light Transmitter Multiphase Sampling Multiphase Transmission Sequential Transmission, via Single Light Parallel Transmission, via Multiple Lights Receiver Processing of Multiphase Transmission Multiphase Transmission with Phase Synchronization Field Receiver Processing of Multiphase Transmission with Phase Synchronization Field Method Flowcharts Multiphase Transmission Multiphase Transmission with Phase Synchronization Field Computer Processor System Wireless Communication Receiver System Computer Program, Apparatus, and Method Embodiments

Light Arrays

FIG. 1A is an illustration of an example light array 100, which may operate according to embodiments described herein. Light array 100 includes LEDs 102 that are spatially-separated from each other in 2-dimensions, but clustered closely together around a center LED 104.

FIG. 1B is an illustration of an example light array 110, which may operate according to embodiments described herein. Array 110 includes a rectangular array of LEDs 112 that are spatially-separated so as to be relatively far apart from each other compared to lights 102 of array 100.

FIG. 1C is an illustration of an example light array 120, which may operate according to embodiments described herein. Array 110 includes a linear array, or line bar, of LEDs 122.

Light Beam Diagram

FIG. 2 is a diagram of an example light array 202 that may operate in accordance with embodiments described herein. FIG. 2 introduces concepts helpful to understanding the embodiments described later. Light array 202 may be configured similarly to any of light arrays 100, 110, and 120, or any other light array including spatially-separated lights. Array 202 includes lights 204 a-204 d implemented to transmit simultaneously a respective one of free-space optical light beams 206 a-206 d to a multi-dimensional or planar light imager/sensor 208, through an imaging lens 210. The terms “light beam” and “light” are use equivalently and interchangeably throughout the ensuing description.

Light imager 208 may include a multi-dimensional charge coupled device (CCD) array including many sensor pixels or light detectors, as is known in the art. Light beams 206 a-206 d are sufficiently spatially-separated from one another as to form corresponding beam images 212 a-212 d, or light spots, on spatially-separated areas of light imager 208. Each of light spots/areas 212 i occupies a position, e.g., an x-y position on a light sensor plane of the light imager, corresponding to a cluster of sensor pixels. Over time, light imager 208 repetitively captures or records, simultaneous light beams 206 i impinging on areas 212 i, to produce a time-ordered sequence 214 of recorded images 216 of light array 202.

Light imager 208 captures the images at a predetermined frame rate of, e.g., approximately 30 frames/second, i.e., every 1/30 seconds. Therefore, sequential images 216 are spaced in time by a frame period equal to an inverse of the frame rate. Sequential images 216 may be processed in accordance with methods described herein.

Light Communication System Using FSOOK

FIG. 3A is a block diagram of an example light communication system 300 useful to introduce the principles of FSOOK modulation and detection/demodulation. System 300 includes a light transmitter 304 to transmit a FSOOK modulated light beam 306 comprising modulated light packets to a light receiver 308, which detects and demodulates the received light. The FSOOK modulated light beam conveys modulated light packets formatted according to protocol light packet definitions.

Light Transmitter

Light transmitter 304 includes a light modulator 309 to intensity modulate a light source 310, a data source 312, and a controller 314 to control the transmitter. Data source 312 provides data 316, such as a message in the form of data bits, to controller 314. Controller 314 includes a memory 318 to store protocol control logic, protocol light packet definitions, and a frame rate F_(fps) in frames per second, which is equal to the inverse of a frame period T_(frame) in seconds (i.e., F_(fps)=1/T_(frame)). The frame rate F_(fps) is an anticipated rate at which light receiver 308 will sample received light, as will be described more fully below in connection with FIG. 3B.

Controller 314 also includes a clock and timer module 319 to generate a master timing signal, and derive from the master timing signal timing outputs used by controller 314 to control transmit light packet start times and durations based on the master timing signal. Based on data 316, the contents of memory 318, and the timing outputs from clock and timer module 319, controller 314 generates commands 320 to cause modulator 309 to modulate light source 310 in accordance with examples described herein.

Modulator 309 includes an FSK modulator 326 and an intensity modulator 327 that together generate a modulation signal 330 to FSOOK modulate light source 310. Controller commands 320 include commands that specify (i) a selected frequency at which FSK modulator is to operate, (ii) a start time at which FSK modulator 326 is to begin generating and outputting the selected frequency, and (iii) a duration (or time period) over which the selected frequency is to be generated. The start time and duration may be graduated in fractions of time period T_(frame), such as 1/1000 of T_(frame). In response to controller commands 320, FSK modulator 326 outputs the selected frequency as an FSK signal 332 beginning at the specified time and duration, such as for an integer number of frame periods, which facilitates detection and demodulation of the frequency at receiver 308. The selected frequencies may include:

-   -   a. a first frequency 328 a F0 (e.g., 120 Hz) indicative of a         logic 0 of a data bit 316 to be transmitted;     -   b. a second frequency 328 b F1 (e.g., 105 Hz) indicative of a         logic 1 of the data bit to be transmitted;     -   c. a third frequency 328 c “HiRate” indicative of a first         start-frame-delimiter to be transmitted. The HiRate frequency is         orders of magnitude greater than frequencies F0, F1, e.g., many         KHz or above. An exemplary HiRate frequency is 25 KHz; and     -   d. a fourth frequency 328 d “Illegal” (e.g., 112.5 Hz, i.e.,         half-way between frequencies F0, F1) indicative of a second         start frame delimiter to be transmitted.

FSK modulator 326 may include a voltage, or digitally, controlled oscillator that generates the above frequency responsive to commands 320. The terms “tone” or “tones” and “frequency” or “frequencies” are used equivalently and interchangeably herein.

FSK modulator 326 may generate each of the frequencies F0, F1, HiRate, and Illegal of FSK signal 332 as a substantially rectangular, or ON-OFF keying, waveform, where ON represents a logic 1 of the FSK waveform, and OFF represents a logic 0 of the FSK waveform. Also, to transmit a data bit, each of frequencies F0 and F1 may extend over multiple frame periods, and may be harmonically related to the frame period such that an integer number, k, of ½ cycles or periods of the rectangular FSK waveform matches the frame period, as is depicted in FIG. 4A (described below). More generally:

-   -   i. representing a logic 0, frequency F0=N×F_(fps); and     -   ii. representing a logic 1, frequency F1=N±0.5F_(fps), where N         is an integer.

Each of the frequencies F0, F1, HiRate, and Illegal, together with the respective number of frames over which they are transmitted, form a light protocol. More specifically, transmitter 304 combines these parameters into the above mentioned modulated light packets formatted in accordance with the light protocol, and then transmits the light packets.

FIG. 4A is an illustration of an example timing diagram of an FSK waveform 404 corresponding to FSK signal 332 in FIG. 3A, where the frame rate F_(fps) is 30 Hz, the bit rate is half the frame rate, i.e., the bit rate is ½ F_(fps)=15 bits-per-second, and N=4. Therefore, each data bit has a duration that is two frames periods, i.e., 2×T_(frame). Therefore, to transmit two consecutive data bits, e.g., a logic 0 followed by a logic 1, controller commands 320 cause FSK modulator 326 to generate first an ON-OFF keying waveform 406 at frequency F0 (e.g., 120 Hz=4×30 Hz) for a time period of two frames to represent the logic 0 data bit, and then an ON-OFF keying waveform 408 at frequency F1 (e.g., 105 Hz=3.5×30 Hz) for a period of two frames to represent the logic 1 data bit. The harmonic relationship between frequencies F0 and F1 and the period of two frames is such that (i) waveform 406 at frequency F0 includes eight full cycles, i.e., k=8, during the data bit period, and (ii) waveform 408 at frequency F1 includes seven full cycles or periods, i.e., k=7, during the second data bit period. In other words, over a bit period, eight cycles of FSK signal 332 represent a logic 0, while seven cycles represent a logic 1.

Intensity modulator 327 intensity modulates light source 310 based on the modulation signal 330, to produce modulated light beam 306. Light source 310 may be an LED that emits light in any of the visible, infrared, or ultraviolet light spectrums. In an embodiment, modulation signal 330 follows the shape of FSK signal 332 and adjusts a current through light source 310 to proportionally adjust an intensity of light 306 emitted by the light source. In this manner, ON-OFF keying of modulation signal 330 causes corresponding ON-OFF keying of the intensity of light 306, such that the intensity closely follows ON-OFF keying waveforms 404, 406 depicted in FIG. 4A. Other intensity modulation embodiments are possible, e.g., light source 310 may not be turned off completely during the OFF cycle of the FSK waveform, and so on. For example, a reduced light intensity (e.g., ½ of maximum intensity) from light source 310 may serve as an alternative for the HiRate frequency. Applying a reduced steady state drive current to the light source 310 will cause the light intensity emitted by the light to be correspondingly reduced. Because other such intensity levels are possible, e.g., light source 310 may not be turned off completely, the intensity levels ON, OFF are more generally represented as intensity levels HIGH, LOW.

Transmitter 304 is depicted with one light 310 for simplicity only. Other embodiments include many lights each driven by a corresponding light modulator, as will be described later in connection with FIG. 6.

Transmit Light Packet Definition

FIG. 4B is an illustration of an exemplary light packet definition 450 or light packet protocol for light packets formatted and transmitted by light transmitter 304. According to light packet definition 450, each light packet includes sequential fields of light, beginning with the SFD, which includes light that is intensity modulated at one of the HiRate and Illegal frequencies for multiple, e.g., four, frame periods. Following the SFD, the light packet conveys a series of consecutive, contiguous message bits B1-B10, each of which may be either a logic 0 or a logic 1. Message bits B1-B10 are each conveyed as light that is intensity modulated at the corresponding FSK frequency F0 (for logic 0) or F1 (for logic 1) for two frame periods, i.e., light that is cyclically keyed to multiple intensity levels (e.g., ON, OFF, or HIGH, LOW) at the FSK frequency indicative of the appropriate bit level (i.e., logic 0 or logic 1).

Light Receiver

FIG. 3B is a block diagram of light receiver 308, according to an embodiment. Light receiver 308 receives the modulated light packets conveyed in modulated light beam 306. In embodiments, light receiver 308 will receive many spatially-separated modulated light beams simultaneously. Light receiver 308 includes a light imager 350 (also referred to as an imager 350) to sample and record received modulated light packets in light beam 306 as images, a detector 352 to detect and demodulate the fields of modulated light in the light packets recorded in the images, and a controller 354 to control the receiver and process the recorded images as described in one or more examples herein.

Imager 350 includes a light sensor 356, e.g., including a 2-dimensional array of light detectors, that repetitively samples light impinging on the light sensor at a predetermined receive sample rate equal to the frame rate, F_(fps)=1/T_(frame), of imager 350 to produce a signal 358. Signal 358 includes a time-ordered sequence of 1-dimensional, or alternatively, 2-dimensional light samples, which form images of an image sequence IS (similar to images 216 depicted in FIG. 2). In other words, the images are formed from the light samples. Accordingly, signal 358 is referred to in terms of both “light samples 358” and “images 358” interchangeable and equivalently. Imager 350 records images 358 in an image memory 355 of the imager.

Light Detector Array

Light sensor 356 may include a 2-dimensional light detector array 359, such as a CCD array, including multiple individual light detectors 360 (also referred to as sensor pixels 360) spatially arranged in M rows by N columns, where M and N may each be in the hundreds or thousands. For convenience, exemplary light detector array 359 is depicted in FIG. 3B as having only 3 rows by 3 columns of light detectors 360. Each of light detectors 360 receives a corresponding one of multiple enable signals 361 generated by an exposure controller 362 of light sensor 356. Enable signals 361 cause light detectors 360 to sample light in a controlled manner, to produce light samples 358 (forming the images), which may be digitized light samples, as will be described more fully below.

An exemplary individual light detector 360(i, j) is depicted in expanded view in FIG. 3B at the bottom right-hand side of the imager block 350. Descriptors (i, j) indicate the row (i) and column (j) positions in array 359, where i=1 . . . M, j=1 . . . N. Light detector 360(i, j) includes a photo-detector 363 followed by an integrate-and-hold (IAH) circuit 364. Photo-detector 363 converts light energy 306 impinging thereon into an electrical signal 365 having a magnitude that follows or represents the intensity of the light energy.

IAH circuit 364 operates as an approximated matched filter to recover samples of the FSK light waveform pulses, such as the pulses of waveforms 406, 408, in the light packets of light beam 306. IAH circuit 364 integrates electrical signal 365 for an integration time t_(int) according to enable signal 361(i, j), to produce a peak integrated signal, also referred to herein as light sample 358(i, j) or sampled light 358(i, j), which is held at the output of the IAH circuit. The process of enabling light detector 360(i, j) to sample light 306 in accordance with enable signal 361(i, j), to produce light sample 358(i, j), is also referred to herein as “exposing light detector 360(i, j), to produce light sample 358(i, j).” Integration time t_(int) may be approximately a half-period or less of the waveforms of frequencies F0, F1, so that light detector 360(i, j) approximately maximally samples light that is intensity modulated at frequencies F0, F1 of FSK waveforms 406, 408 (for logic levels 0, 1).

An exemplary enable signal waveform “ES” of enable signal 361(i, j) is depicted at the bottom of FIG. 3B. Enable signal 361(i, j) (e.g., waveform ES) may include a series of enable pulses 368 spaced in time from each other by frame period T_(frame), i.e., the enable pulses have a pulse repetition rate equal to the frame rate F_(fps)=1/T_(frame) of image sensor 356. Each of enable pulses 368 has a pulse width equal to t_(int) to enable IAH circuit 364 to integrate energy over the pulse width, and hold peak integrated signal 358(i, j) at the output until a next pulse in the series of pulses causes the IAH to resample its input. Enable pulses 368 establish and represent a receive sample timeline of light receiver 308. The receive sample timeline defines the series of times at which receiver 308 samples light 306. In this way, light detector 360(i, j) samples light energy 306 impinging on position (i, j) of light detector array 359 at frame rate F_(fps), to produce sampled light energy as a series of light samples represented at 358(i, j) coinciding with pulses 368. Each of light detectors 360 may simultaneously sample light energy 306, to produce simultaneous light samples 358(1-M, 1-N) represented in signal 358.

Global and Line Array Exposure Modes

Exposure controller 362 generates enable signals 361 in any number of ways to implement different exposure modes of light detector array 359, as is now described.

Exposure controller 362 may expose array 359 (i.e., enable light detectors 360 to sample light 306 in accordance with enable signals 361, to produce light samples 358) in either a global exposure mode or, alternatively, in a sequential line exposure mode. In the global exposure mode, exposure controller 362 generates enable signals 361 so that their respective series of enable pulses 368, i.e., respective integration periods t_(int), coincide in time with each other, i.e., occur at the same time. The result is that all of light detectors 360 are exposed at the same time, i.e., they all sample light 306 at the same time, once every frame period T_(frame), to produce a time-spaced sequence of 2-D images represented in images 358 (which represents all light samples 358(i, j), i=1 . . . M, j=1 . . . N), as represented in FIG. 3B by image sequence IS. Each image in the sequence of images IS includes a 2-D array of light samples corresponding to the 2-D array of light detectors 360.

In the line exposure mode, exposure controller 362 may generate enable signals 361 to expose spatially-successive lines, e.g., successive rows or successive columns, of light detectors 360 one after the other, e.g., one at a time, in a time sequence. For example, exposure controller 361 may generate enables signals 361 so as to expose:

-   -   a. all of light detectors 360 across row i−1 (i.e., all of the N         light detectors 360(i−1, 1-N)) at a same time t−τ; then     -   b. all of light detectors 360 across row i at a same time t;         then     -   c. all of light detectors 360 across row i+1 at a same time t+τ,         and so on.

This produces spatially-successive lines of sampled light, spaced in time at sequential times t−τ, t, t+τ, corresponding to light detector rows i−1, i, i+1, and so on. This type of exposure is also referred to as “rolling shutter exposure” because the exposure may be thought of as being implemented using a camera shutter one line of light detectors wide (i.e., that is only wide enough to expose one line of light detectors at a time), that “rolls” or scans sequentially across spatially-successive lines (e.g., the rows or columns) of light detectors in a given direction (e.g., up/down, left/right), to thereby sequentially expose the spatially-successive lines of light detectors. In an embodiment, exposure controller 362 sequentially exposes the spatially-successive lines of light detectors at a rate (referred to as a “line exposure rate”) that is greater than both frequencies F0, F1 of the FSK waveforms representing logic levels 0, 1 in transmitted light packets. The line exposure rate is equal to 1/τ.

In a variation of the above-described line exposure mode, the enable signals 361 may be generated to be slightly offset in time but overlapping, so that the exposure of each line time-overlaps the exposure of the spatially-successive line. For example, row i−1 begins its exposure at a time t_(i−1), and while being exposed (e.g., before time t_(int) expires for row i−1), row i begins its exposure, and while being exposed (e.g., before time t_(int) expires for row i), row i+1 begins its exposure, and so on. This variation of the line exposure mode results in time spaced lines of sampled light corresponding to light detector rows i−1, i, i+1, but with overlapping exposure times for successive rows.

FIG. 3C is a block diagram of light imager 350 including light sample digitizing modules, according to an embodiment. Light detectors 360 provide corresponding sampled outputs 380 to a light detector (or pixel) scanning analog-to-digital converter (ADC) 382 that sequentially scans across each of the light detectors and digitizes its corresponding sampled output, to produce sequential, digitized sampled outputs 384. A demultiplexer 386 converts the sequential, digitized sampled outputs into an array of digitized, sampled outputs representative of images 358. Use of scanning ADC 382 and demultiplexer 386 reduces the number of ADCs that might otherwise be required to digitize all of the sampled outputs 380 in parallel.

Detector

Detector 352 includes a beam position determiner module 370 a, and a SFD detector/demodulator module 370 b (collectively referred to as “modules 370” and “modules 370 a, 370 b”), which cooperate to process the sequence of images stored in memory 355, namely to:

-   -   a. determine a position of each beam recorded in the images,         such as an x, y center coordinate of the beam in each image         (using beam position determiner 370 a); and     -   b. from the modulated light recorded at the determined beam         positions, both detect any delimiters (SFDs) and demodulate any         data bits conveyed by that recorded light (using         detector/demodulator 370 b).

As described above, light detectors 360 sample FSK waveform pulses in light 306, such as the pulses of waveforms 406, 408 at frequencies F0, F1 (representing logic levels 0, 1), and provide the resulting samples 358 to modules 370 a, 370 b, e.g., in a sequence of 1-dimensional or 2-dimensional images IS.

To detect a beam position, beam position determiner 370 a raster scans the full area of each image in the sequence of images (e.g., in image sequence IS) stored in memory 355, e.g., first, second, third, and fourth sequential images, and so on, in search of recorded light energy that has a correlated position across the sequence of images. In other words, a beam position is determined when beam position determiner 370 a detects a spot of modulated light, i.e., modulated light energy, centered on the same position, e.g., an x, y position corresponding to a row, column position, in each of the sequential images. Beam positions for multiple, spatially-separated, simultaneously recorded beams may be determined in this manner.

From each determined position, SFD detector/demodulator 370 b associates corresponding light samples 358, over multiple recorded images, to one of: a demodulated data bit level, i.e., logic 0 or logic 1; a demodulated data delimiter; and a detected SFD.

FIG. 5 is a light amplitude/intensity (y-axis) vs. time (x-axis) diagram helpful in understanding how SFD detector/demodulator 370 b associates light samples 358 with demodulated data bits. In the example of FIG. 5, exemplary light 306 conveys a logic 0 followed by a logic 1, i.e., the light is intensity modulated at FSK frequencies F0 and F1 for first and second bit periods, i.e., where each bit period is twice frame period T_(frame). On the diagram of FIG. 5, light intensity values of 1, −1 correspond to light intensity values of ON, OFF, (or HIGH, LOW) respectively. Assuming light 306 impinges on a given one of light detectors 360, then that light detector samples light 306 once every frame period T_(frame) (i.e., twice per bit period), in accordance with a receiver sample timeline, to produce a sequence of time-spaced light samples S1, S2, S3, and S4, with an arbitrary sample phase relative to the bit periods. If light 306 is sampled once per frame period while the FSK waveforms at frequencies F0, F1 may produce 3 or 4 full cycles per frame period, the FSK waveforms are under-sampled compared to the Nyquist rate, i.e., two samples per FSK waveform cycle.

During the first bit, or logic 0, period, the frequency/timing relationship between the 120 Hz ON-OFF keying of light 306 and the light sample spacing, i.e., the frame period T_(frame), causes consecutive light samples S1 and S2 to be in the same intensity state, i.e., at the same level (either ON/HIGH). In the example of FIG. 5, consecutive samples S1 and S2 are both ON. However, the absolute level, e.g., ON or OFF, depends on the sample phase of the receiver sample timeline. Therefore, if two consecutive light samples indicate consecutive same ON-ON or OFF-OFF states, then detector/demodulator 370 b associates this condition with, and demodulates, a logic 0.

During the second bit, or logic 1, period, the frequency/timing relationship between the 105 Hz ON-OFF keying of light 306 and the light sample spacing causes successive light samples S3 and S4 to toggle between states either (ON then OFF, or OFF then ON). In the example of FIG. 5, consecutive samples S3 and S4 transition from ON to OFF. However, in other examples, with different sample phases of the receiver sample timeline, S3 and S4 may toggle from OFF to ON. Therefore, if two consecutive light samples indicate a state transition ON-OFF or OFF-ON, then detector/demodulator 370 b demodulates a logic 1.

The above-described exemplary demodulation of FSOOK modulated light is based on under-sampling the FSK waveform. Therefore, such demodulation is referred to herein as under-sampled FSOOK (UFSOOK) demodulation.

Modules 370 a, 370 b also monitor light samples (i.e., images) 358 to detect light modulated with the Illegal frequency, as an indicator of a SFD associated with a light packet. As mentioned above in connection with demodulated data bits, the relationships between the frame period and the frequencies F0, F1 respectively causes detected light in two consecutive images always to be either in the same state, or in different states. However, the relationship between the frame period and the Illegal frequency causes detected light to toggle ON and OFF over four consecutive images in an ON-OFF pattern that cannot occur when the light is modulated at frequencies F0, F1. More specifically, if the light samples indicate any of patterns ON-ON-OFF-OFF, OFF-OFF-ON-ON, ON-OFF-OFF-ON, and OFF-ON-ON-OFF over four consecutive images, then modules 370 a, 370 b detect the Illegal frequency associated with the data delimiter.

Modules 370 a, 370 b also monitor light samples 358 to detect light modulated with the HiRate frequency, as an indicator associated with the SFD. An SFD modulated at the HiRate frequency may be more readily detected relative to an SFD modulated at the Illegal frequency when embedded with message data bits (e.g., logic 0, 1) because the HiRate frequency is more easily distinguished from the logic 0, 1 FSK frequencies than the Illegal frequency, which falls between those frequencies.

While light detectors approximately maximally detect frequencies F0, F1 in the modulated light, i.e., produce a near maximum amplitude output in response to the matched frequency, the integration time of the light detectors is too long to respond fully to the much greater HiRate frequency. Therefore, light detectors 360 are suboptimal energy detectors/samplers of the HiRate frequency, and provide an average, e.g., approximately ½ maximum, amplitude output (i.e., sampled output) in response to the HiRate frequency. Therefore, modules 370 a, 370 b detect the SFD in modulated light beam 306 when light detectors 360 provide the average, lesser amplitude outputs in response to sequential images. Similarly, in a transmit embodiment in which a reduced light intensity serves as an alternative for the HiRate frequency, light detectors 360 provide an average, lesser amplitude indicative of the reduced light intensity.

From recorded sampled light at a determined position in a sequence of images, modules 370 a, 370 b demodulate frequencies F0, F1 into data bit logic levels, detect the HiRate frequency, and detect the Illegal frequency associated with the SFD. Modules 370 a, 370 b also detect the number of frames over which each of the above mentioned frequencies extend. In this way, detector 352 deconstructs or determines the modulated light packets conveyed in the recorded light beam(s). Modules 370 a, 370 b pass such information to controller 354 over a bidirectional interface 374. For example, over interface 374, modules 370 a, 370 b indicate detected SFDs from recorded light packets to controller 354, and provide demodulated data bits from the light packets to the controller.

Controller

Controller 354 includes a memory 376 to store control logic, protocol light packet definitions, and a frame period. Controller 354 provides light packet protocol definitions to detector 352 over interface 374. Based on the information from detector 352 and the contents of memory 376, controller 354 operates and controls receiver 308. Controller 354 also controls imager 350 over interface 374, e.g., the controller may command exposure controller 363 to operate in either of the global exposure mode or the line exposure mode. Controller 354 is also referred to herein as a “protocol processor.”

Multi-Light Transmitter

FIG. 6 is a block diagram of an example multi-light transmitter 640 to transmit light packets. Light transmitter 640 includes an array or group of spatially-separated lights 642, which may be spatially-arranged in either 1-dimension or in 2-dimensions.

Transmitter 640 includes light modulators 648, which may be implemented similarly to modulator 309 in FIG. 3, each to modulated light from a corresponding one of lights 642. Transmitter 640 may include a controller 650, including memory and one or more clock and timer circuits similar to those of controller 314. Controller 650 receives multiple parallel data inputs (e.g., one per light modulator) from data sources not shown, and generates modulator commands 651 in parallel to control multiple modulators 648, similar to the manner in which commands 320 control modulator 309. In an alternative embodiment, controllers, such as controller 314, may be incorporated into each of modulators 648 separately.

In response to commands 651, modulators 648 modulate their corresponding lights 642 to transmit their respective light packets in spatially-separated light beams 652 according to the light packet definition of FIG. 4B, to convey data bits received over the data inputs. In response to commands 651, modulators/lights 648/642 may transmit their respective light packets with any number of different inter-packet timing relationships. For example, modulators/lights 648/642 may transmit their respective light packets simultaneously with each other. Alternatively, the light packets may be transmitted in a serial manner, one after the other. Alternatively, the light packets may be transmitted with their respective start times offset slightly with respect to each other. Any combination of such inter-packet timing relationships is possible.

Multi-Phase Sampling

In the examples described above, light transmitter 304 transmits light packets modulated to convey sequential fields, including an SFD, followed by a message including a series of data bits. A data bit is conveyed at a transmit bit rate as an FSK frequency representing a value of the bit, where the transmit bit rate and the FSK frequency are harmonically related. Ideally, light receiver 308 samples the light packets when received at the receive sample rate, F_(fps), based on a receive sample timeline (e.g., timeline ES in FIG. 3B) that is also harmonically related, and thereby synchronized, to the transmit bit rate and the FSK frequency.

In practice, however, the transmit bit rate (and FSK frequency) and the receive sample rate may not be synchronized, i.e., they are asynchronous, because of differences between transmitter and receiver clocks/timers. When the transmit bit rate (and FSK frequency) and the receive sample rate are asynchronous, then, at the receiver, the FSK waveforms corresponding to the transmitted data bits and the receive sample times gradually “slip” or “drift” by each other. As the FSK waveforms gradually slip by the sample times of the receive sample timeline, occasionally, a sample time will coincide with an FSK waveform transition from ON-to-OFF or OFF-to-ON. That is, the sample integration time t_(int) of light detectors 360 may coincide with the FSK waveform transition. This may result in an indeterminate level of a light sample, i.e., a level reflecting that the light is neither fully ON nor fully OFF, and yield an erroneous bit level determination, which may increase a packet or bit error rate of the light receiver.

Assuming such asynchronous timing between the transmit bit rate and the receive sample rate, techniques that result in multiphase sampling of received light packets, i.e., that result in multiple sampling phases at the receiver, reduce a likelihood that any given one of the multiple sampling phases will coincide with FSK waveform transitions, and thereby reduce the likelihood of bit demodulation errors. Therefore, multiphase sampling advantageously mitigates the deleterious effects mentioned above that arise from asynchronous transmit and receive timing. Accordingly, described below are multiphase sampling embodiments, including two multiphase transmission embodiments and two multiphase transmission embodiments that each utilize a phase synchronization field, that improve the bit error rate performance of an asynchronously sampling light receiver.

Multiphase Transmission

Two multiphase transmission embodiments, namely, sequential multiphase transmission and parallel multiphase transmission, are now described, generally. In both embodiments, a light transmitter, e.g., one of transmitters 304 and 640, transmits multiple light packets to light receiver 308. Each of the multiple light packets includes a same, i.e., redundant, message comprising a series of bits, each bit being represented as light that is intensity modulated over a bit period at a frequency indicative of the bit, in the manner described above. The light transmitter issues controller commands 320/651 to vary transmit start-times of the multiple same messages (carried in the multiple light packets) relative to each other and an anticipated, receive sample timeline established by light receiver 308 that is asynchronous to the transmit bit rate/period and the frequency. The different message start-times establish slightly different receive sample phases of the receive sample timeline, one for each of the same messages.

Light receiver 308 receives and samples the multiphase (or time-offset) light packets based on the asynchronous receive sample timeline, to produce samples for each received message that coincide in time with the corresponding one of the different sample phases established for that message. In other words, due to the slightly different start-times of each message, receiver 308 samples each message at a slightly different phase of the receive sample timeline. This produces phase-offset samples for each of the same messages. Receiver 308 demodulates each of the received same messages based on the samples for that message, which may include random demodulation errors due to the asynchronous sampling in the receiver. Then receiver 308 constructs one best message based on the multiple demodulated messages, i.e., the receiver combines the multiple demodulated messages into a single best message.

The two embodiments of multiphase transmission are now described more fully below.

Sequential Multiphase Transmission Via Single Light

A first embodiment of multiphase sampling, referred to herein as sequential multiphase transmission via a single light/transmit source, or simply sequential multiphase transmission, is described with reference to FIG. 7. FIG. 7 is a timing diagram of exemplary transmit light packets 704 transmitted by light transmitter 704, in accordance with sequential multiphase transmission. Light packets 704 include contiguous, sequential light packets 704A-704B, each formatted, in accordance with commands 320 of transmitter 304, to include a variable length SFD followed by a message comprising a series of data bits. Each of the light packets 704 includes (i) an SFD having a time duration that is different from all of the other SFDs, and (ii) a message, i.e., series of data bits, that is the same across the messages. Light packets 704 are transmitted contiguously one after the other, in sequence.

As depicted in FIG. 7, successive SFDs SFD1, SFD2, and SFD3 (of respective light packets 704A, 704B, and 704C) have respective time durations T, T+Δ, and T+Δ that increase monotonically, from the initial time duration of the SFD1, by an increment of time Δ. Time increment Δ may be approximately greater than or equal to t_(int), such as twice t_(int). The result is that successive data messages have respective successive start times t1, t2 and t3 separated in time from each other, i.e., one from the next, by successive time separations, e.g., 708A, 708B, that also increase incrementally by time increment Δ. The effect is to introduce multiphase offsets between the successive repeated data messages with respect to an arbitrary, fixed receive sample timeline.

Parallel Multiphase Transmission Via Multiple Light Sources

A second embodiment of multiphase sampling, referred to herein as parallel multiphase transmission via multiple lights/transmit sources, or simply parallel multiphase transmission, is described with reference to FIG. 8. FIG. 8 is a timing diagram of exemplary transmit light packets 804 transmitted by a light transmitter, such as multi-light transmitter 640 in accordance with parallel multiphase transmission. Light packets 804 include parallel, time-staggered (i.e., phase-offset) light packets 804A-804B, each transmitted by a respective one of lights 642(1)-642(3) of multi-light transmitter 640. Each of the light packets 804 includes (i) a fixed length SFD having a time duration that is the same as that of all of the other light packets (in contrast to the variable length SFDs of FIG. 7), and (ii) a message, i.e., series of data bits, that is the same across the messages.

Successive packets 804A, 804B, and 804C have respective successive start times (i.e., the times at which the SFDs start) that increase monotonically from a reference time 0 by an increment of time Δ. Since the SFDs all have the same time duration, the successive data messages of successive packets 804A, 804B, and 804C also have respective successive start times that increase monotonically from time 0. The effect is to introduce multiphase offsets between the time-overlapping, time-staggered, successive repeated data messages.

Receive Processing of Multiphase Transmission

Light receiver 308 processes sequential multiphase light packets 704 or parallel multiphase light packets 804 as described above in connection with FIGS. 3B and 5. That is, at receiver 308, sequential multiphase light packets 704, or parallel multiphase light packets 804, are each asynchronously sampled and demodulated to recover their repeated data messages, each of which includes the same series of bits across the repeated light packets. Due to the multiphase offsets of the data messages introduced on the transmit end, at receiver 308, each of the messages is periodically sampled (i.e., at the receiver frame rate) across its duration at slightly different or offset sample times relative to the other messages, thereby increasing the likelihood that most of the sample times will avoid FSK waveform edge transitions and, thus, demodulated bit errors. In this manner, light receiver 308 recovers or demodulates multiple copies of the same data message, each of which may include one or more demodulated bit errors due to the asynchronous sampling. Light receiver 308 combines the multiple copies using, e.g., bit voting, to construct a single, best message representative of the transmitted messages, as will be described more fully below.

FIG. 9 is an example timing diagram 900 at light receiver 308 corresponding to the sequential and parallel multiphase transmission embodiments. The example of FIG. 9 assumes that the multiphase light packets transmitted and then received at receiver 308 include three light packets carrying the same data message comprising a series of data bits 1-0-1. Each of the three light packets conveys its same series of data bits 1-0-1 as three light intensity modulated FSK waveforms at frequencies F1-F0-F1 corresponding to the data bits. In FIG. 9, the three transmitted data messages (where each of the data messages includes the three FSK waveforms representing 1-0-1) are superimposed in time, one on top of the other, so as to be represented by a single, superimposed waveform 902 representing the series of bits 1-0-1.

Such waveform/time superposition of the transmitted waveforms in FIG. 9 effectively removes the multiphase time offsets introduced between the data messages on the transmit end, and, when viewed from the timing perspective of the receiver, introduces those time offsets into the sample times established by the single receive sample timeline for the three superimposed light packets. Accordingly, as depicted in FIG. 9, the receiver samples:

-   -   a. the first (superimposed) light packet at a first sample         phase, to produce a first series of sample times 904-1, 904-2,         904-3, and so on;     -   b. the second (superimposed) packet at a second sample phase, to         produce a second series of sample times 906-1, 906-2, 906-3, and         so on, slightly offset in time with respect to the first sample         phase; and     -   c. the third packet at a third sample phase, to produce a third         series of sample times 908-1, 908-2, 908-3, etc., slightly         offset in time with respect to the first and second series of         sample times.

Each phase/series of samples, e.g., 904, is slightly offset in time/phase with respect to the succeeding phase/series of samples, e.g., 906, by the time increment or offset A established at the light transmitter. As seen in FIG. 9, some of the samples fall close to, or coincide with, FSK waveform edge transitions, which may result in some erroneous bit level determinations. However, other samples avoid the FSK waveform transitions because they are offset in time from those that fall close to, or coincide with, the transitions, and thereby result in correct bit level determinations.

The receiver recovers/demodulates a separate message corresponding to each of the three phases/series of samples (e.g., a first demodulated message based on samples 904, a second demodulated message based on samples 906, and a third demodulated message based on samples 908), to produce three separate demodulated messages, and constructs a final, or best, message based on the three demodulated messages.

FIG. 10 is a diagram of an example 1000 of multiphase transmission and reception, in which transmitter 304 or 640 transmits three multiphase light packets each carrying the same 10-bit data message P_(sent) depicted in FIG. 10, with multiphase offsets. Light packets P_(sent) may be transmitted in accordance with either sequential or parallel multiphase transmission embodiments described above.

Receiver 308 receives and samples the three light packets with respective sample phases 1, 2, and 3 and demodulates the light packets according to each of the three sample phases, to produce corresponding demodulated packets P1, P2, and P3, each including 10-bits corresponding to the 10-bits of transmitted light packet P_(sent), as depicted in FIG. 10. Several demodulation bit errors result when asynchronous receive samples coincide with FSK waveform transitions.

Receiver 308 performs forward error correction (FEC) on the three demodulated packets P1-P3, to construct a best packet P_(fec). In an embodiment, the FEC includes majority bit voting across corresponding bits of the three demodulated light packets P1-P3, to produce best packet P_(fec).

Multiphase Transmission with Phase Synchronization Field

Described below are two further embodiments of sequential and parallel multiphase transmission that take advantage of a feature or field of an SFD, referred to herein as a phase synchronization field (PSF), to improve multiphase performance. Accordingly, these embodiments are referred to herein as multiphase (sequential and parallel) transmission with phase synchronization field. Use of the PSF can improve forward error correction performance when a duty cycle of transmitted FSOOK modulated light deviates either below or above 50%. For example, the exemplary FSOOK FSK waveforms of FIGS. 4A and 5 are depicted as having approximately 50% ON-OFF duty cycles. However, a light transmitter may transmit FSOOK modulated light at duty cycles well below and above 50%. This is because pulse width modulation (PWM) may be used in the light transmitter to vary a duty cycle of the transmitted light from near 0% (very dim light) to near 100% (very bright light), to cause the modulated light to appear dimmer or brighter.

When the duty cycle is varied from 50%, referred to as pulse-width dimming, the amount of transmitted light energy in each of the FSOOK modulated data bits decreases; i.e., the signal to noise ratio of the data bit decreases. For example, a 10% FSOOK waveform duty cycle, in which light is ON 10% of the time and OFF 90% of the time, represents a substantial reduction in the transmitted data bit signal to noise ratio compared to a 50% FSOOK waveform duty cycle. The decrease in duty cycle also correspondingly increases a likelihood that a light receiver sample timeline in a light receiver will cause the receiver to erroneously sample the OFF portions of the FSOOK waveform instead of ON portions of the FSOOK waveform, resulting in further demodulated bit errors. Processing the PSF in the SFD, in addition to repetitive time-offset message transmission (as described above in connection with FIGS. 7-10), can improve the bit error rate performance to combat the reduced signal to noise ratio and possible sampling errors caused by such pulse-width dimming

FIG. 11 is an exemplary light packet definition 1100 for FSOOK modulated light packets (corresponding to light packet definition 450 of FIG. 4B) used in multiphase transmission with phase synchronization field, according to an embodiment. Light packet definition 1100 provides an expanded or more detailed view of the SFD of light packet definition 450, according to an embodiment.

Light packet definition 1100 includes sequential fields of light modulated to indicate (i) the SFD, including a HiRate frequency field (HRF) followed by a predetermined PSF, and (ii) a payload or series of data bits, following the PSF. In the example depicted in FIG. 1100:

-   -   a. the HRF portion of the SFD comprises 2 frame periods of the         HiRate frequency, e.g., 25 KHz;     -   b. the predetermined PSF portion of the SFD is represented as a         logic 1 (2 frame periods of FSK frequency F1, e.g., 105 Hz); and     -   c. the data message, or payload, of data bits includes an         exemplary series of data bits 1, 0, 1 represented by FSOOK         frequencies F1, F0, F1, e.g., 105 Hz, 120 Hz, 105 Hz.

In other embodiments, the PSF may be represented as multiple sequential logic 1s. The PSF and the HRF may be considered a composite SFD that includes both the HiRate and PSF frequencies.

FIG. 12A is a timing diagram of exemplary transmit light packets 1204 transmitted by light transmitter 304, in accordance with sequential multiphase transmission with PSF. Multiple light packets 1204 include contiguous, sequential light packets 1204A-1204C, each formatted, in accordance with commands 320 of transmitter 304, to include a variable length HRF followed by the PSF, and a message comprising a series of data bits following the PSF. Because the HRF forms part of the SFD, the variable length HRF correspondingly results in variable length SFD. Each of the light packets 1204 includes (i) an HRF having a time duration that is different from all of the other HRFs. Light packets 1204 are transmitted contiguously one after the other, in sequence.

As depicted in FIG. 12, successive SFDs SFD1, SFD2, and SFD3 (of respective light packets 1204A, 1204B, and 1204C) have respective time durations T, T+Δ, and T+2Δ that increase monotonically, from the initial time duration of the SFD1, by an increment of time Δ. Time increment Δ may be approximately equal to or greater than t_(int), such as twice t_(int). The result is that successive PSFs have respective successive start times t1, t2 and t3 separated in time from each other, i.e., one from the next, by successive time separations, e.g., 1208A, 1208B, that also increase incrementally by time increment Δ. The effect is to introduce multiphase offsets between the successive repeated PSFs and their corresponding data messages with respect to an arbitrary, fixed receive sample timeline.

FIG. 12B is a timing diagram of exemplary transmit light packets 1214 transmitted by a light transmitter, such as multi-light transmitter 640 in accordance with parallel multiphase transmission with PSF. Light packets 1214 include parallel, time-staggered (i.e., phase-offset) light packets 1214A-12144C, each transmitted by a respective one of lights 642(1)-642(3) of multi-light transmitter 640. Each of the light packets 804 includes (i) a fixed length SFD/HRF having a time duration that is the same as that of all of the other light packets (in contrast to the variable length SFDs/HRFs of FIG. 12A), a PSF following the SFD, and (ii) a message, i.e., series of data bits, that is the same across the messages, following the PSF.

Successive packets 1214A, 1214B, and 1214C have respective successive start times (i.e., the times at which the SFDs start) that increase monotonically from a reference time 0 by an increment of time Δ. Since the SFDs all have the same time duration, the successive PSFs and their corresponding data messages of successive packets 1214A, 1214B, and 1214C also have respective successive start times that increase monotonically from time 0. The effect is to introduce multiphase offsets between the time-overlapping, time-staggered, successive repeated data messages.

Receive Processing of Multiphase Transmission with Phase Synchronization Field

In the multiphase transmission with PSF embodiment, light receiver 308 has an a prior knowledge of the predetermined PSF, e.g., logic 1. For example, the predetermined PSF may be stored in a memory of light receiver 308. Light receiver 308 processes sequential multiphase light packets 1204 or parallel multiphase light packets 1214 conveying the PSF similar to the way in which the receiver processes light packets 704 or 708 that do not convey the PSF, except that the receiver selects sample time phases based on whether the PSF is detected/demodulated correctly, as described below.

Light receiver 308 asynchronously samples received sequential multiphase light packets 1204, or parallel multiphase light packets 12144. Due to the multiphase offsets of the PSFs introduced on the transmit end, at receiver 308, each of the PSFs is periodically sampled (i.e., at the receiver frame rate) across the duration of the PSF at slightly different or offset sample times relative to the other PSFs. In this manner, light receiver 308 recovers or demodulates multiple copies of the PSF, one per received light packet. One or more of the demodulated PSFs may include a demodulated bit error due to the asynchronous sampling. Light receiver 308 determines which of the demodulated PSFs are correct, i.e., which of the demodulated PSFs match the predetermined PSF, e.g., logic 1. Light receiver 308 assumes that a correctly demodulated PSF indicates that the sampled light packet from which the PSF was demodulated is properly time-aligned, throughout the duration of the light packet, with the receive sample timeline so as to avoid the sample errors discussed above (i.e., sample times aligned with waveform transitions). In other words, receiver 308 assumes that if the PSF in a given light packet is sampled and demodulated correctly, then so too is the data message in that light packet sampled and demodulated correctly.

Accordingly, light receiver 308 declares as valid all demodulated data messages corresponding to light packets from which a correct PSF was demodulated. If multiple demodulated PSFs are valid, then light receiver 308 declares as valid each of the multiple demodulated data messages corresponding to a correct demodulated PSF. In the case of multiple valid demodulated data messages, light receiver 308 performs forward error correction, such as bit voting, across all of the valid demodulated messages to construct a best—corrected—data message, as described above in connection with FIG. 10, for example. If only one demodulated data message is declared valid, then that message is the corrected data message.

Method Flowcharts

Multiphase Transmission

FIG. 13 is a flowchart of an example method 1300 summarizing multiphase transmission embodiments described above in connection with FIGS. 7-10. For convenience, the flowchart combines a transmit method 1305-1310 with a receive method 1315-1330. However, it is understood that the transmit and receive methods may be performed separately and independently from each other.

1305 includes transmitting multiple light packets each formatted to include a same message comprising a series of bits, each bit represented as light that is intensity modulated over a bit period at a frequency indicative of the bit.

1310 includes varying transmit start-times of the multiple messages relative to each other and a receiver sample timeline that is asynchronous to the bit period and the frequency, to thereby establish different sample phases of the sample timeline, one for each of the messages. In other words, the varying transmit start-times establish different transmit time offsets for the messages, which results in different sampling phases of the sample timeline.

1315 includes receiving the multiple transmitted light packets at different times according to the different start-times.

1320 includes sampling the received multiple light packets based on the asynchronous sample timeline, to produce samples for each received message that coincide in time with the one of the different sample phases established for that message.

1325 includes demodulating each of the sampled messages, to produce multiple demodulated messages.

1330 includes constructing a series of data bits representative of the same message based on the multiple demodulated messages.

In a parallel multiphase transmission embodiment, 1305 and 1310 include transmitting the multiple light packets as multiple, spatially separated light packets so that all of the light packets partially overlap in time, and varying the start-times of the multiple light packets and, as a result, the start-times of their respective messages, so that the messages have successive start-times, so as to establish the different sample phases as successive sample phases. In this embodiment, 1315 and 1320 include receiving and sampling multiple spatially-separated received light packets.

In a sequential multiphase transmission embodiment, wherein each of the multiple light packets includes sequential fields including a delimiter having a variable time duration followed by the same message, 1305 and 1310 include transmitting the multiple light packets as successive contiguous light packets, and varying the time durations of the successive delimiters with respect to each other and the sample timeline, to thereby establish the different sample phases.

Multiphase Transmission with Phase Synchronization Field

FIG. 14 is a flowchart of an example method 1400 summarizing multiphase transmission with phase synchronization field described above in connection with FIGS. 10, 11, 12A, and 12B. For convenience, the flowchart combines a transmit method 1405-1410 with a receive method 1415-1440. However, it is understood that the transmit and receive methods may be performed separately and independently from each other.

1405 includes transmitting multiple light packets, in sequence or in parallel, each formatted to include a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies.

1410 includes varying transmit start-times of the multiple light packets relative to each other to permit a receiver to sample each light packet at a different phase of a fixed sample timeline, defining when the receiver is to sample the light packets, that is asynchronous to the bit period and the frequency.

1415 includes receiving the multiple transmitted light packets.

1420 includes sampling the received multiple light packets based on the asynchronous fixed sample timeline, to produce samples for each received message that coincide in time with a corresponding one of the different phases.

1422 includes detecting the HRF in each of the sampled light packets.

1425 and 1430 include demodulating the PSF portion of the SFD and the corresponding data message from each of the sampled light packets, respectively.

1435 includes declaring a demodulated data message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.

1440 includes, if at least two of the demodulated data messages are declared valid, then performing forward error correction on the at least two valid demodulated data messages to construct a corrected demodulated data message.

Computer Processor System

FIG. 15 is a block diagram of an example computer processor system 1500 configured for multiphase sampling processing, including light transmitter processing such as light modulation, etc., and light receiver processing such as demodulation, etc., in accordance with examples described herein. In FIG. 15, various transmit and receive components/modules of computer system 1500 are depicted together for descriptive convenience. It is understood that various ones of the components/modules may reside in separate light transmitter and light receiver systems, as appropriate.

Computer system 1500 may include one or more instruction processing units, illustrated here as a processor 1502, which may include a processor, one or more processor cores, or a micro-controller.

Computer system 1500 may include memory, cache, registers, and/or storage, illustrated here as memory 1504.

Memory 1504 may include one or more non-transitory computer readable mediums encoded with a computer program, including instructions 1506.

Memory 1504 may include data 1508 to be used by processor 1502 in executing instructions 1506, and/or generated by processor 1502 during execution of instructions 1506. Data 1508 includes protocol information 1511, including light packet protocol definitions, frame periods, and so on, and recorded images 1513 from an imager, such as a camera, which may be received through the I/O interface.

Instructions 1506 include instructions 1510 a for light receiver (RX) processing of recorded images in accordance with one or more multiphase sampling embodiments (with and without use of the PSF), as described in one of the examples above. Instructions 1510 a include instructions for implementing a detector 1514, a receiver control/protocol processor 1516, and an exposure controller 1524, as described in one or more examples above. Detector instructions 1514 further include instructions for implementing a detector/demodulator 1522 such as a FSOOK detector/demodulator, and a beam position determiner 1526, as described in one or more examples above.

Instructions 1506 may also include instructions 1510 b for a light transmitter operating in accordance with one or more multiphase sampling embodiments described above. Instructions 1510 b include instructions 1517 for controlling the transmitter, and 1518 for implementing a modulator, such as a FSOOK modulator, as described in one or more examples above.

The instructions described above and depicted in FIG. 15 are also referred to herein as processing modules to implement the functions described in one or more examples above.

Wireless Communication Receiver System

FIG. 16 is a block diagram of an example system 1600 including a system or apparatus 1602 to sample and record light beams 1602 a as a sequence of images, and process the recorded images in accordance with one or more multiphase sampling embodiments.

System 1602 may be implemented as described in one or more examples herein. System 1602 may be implemented as a light receiver as described in one or more examples herein. System 1600 may include a processor 1604.

System 1600 may include a communication system, including a transceiver, 1606 to interface between system 1602, processor system 1604, and a communication network over a channel 1608. Communication system 1606 may include a wired and/or wireless communication system.

System 1600 or portions thereof may be implemented within one or more integrated circuit dies, and may be implemented as a system-on-a-chip (SoC).

System 1600 may include a user interface system 1610 to interface system 1610.

User interface system 1610 may include a monitor or display 1632 to display information from processor 1604.

User interface system 1610 may include a human interface device (HID) 1634 to provide user input to processor 1604. HID 1634 may include, for example and without limitation, one or more of a key board, a cursor device, a touch-sensitive device, and or a motion and/or imager. HID 1634 may include a physical device and/or a virtual device, such as a monitor-displayed or virtual keyboard.

User interface system 1610 may include an audio system 1636 to receive and/or output audible sound.

System 1600 may further include a transmitter system to transmit signals from system 1600.

System 1600 may correspond to, for example, a computer system, a personal communication device, and/or a television set-top box.

System 1600 may include a housing, and one or more of communication system 1602, digital processor system 1612, user interface system 1610, or portions thereof may be positioned within the housing. The housing may include, without limitation, a rack-mountable housing, a desk-top housing, a lap-top housing, a notebook housing, a net-book housing, a tablet housing, a set-top box housing, a portable housing, and/or other conventional electronic housing and/or future-developed housing. For example, communication system 1602 may be implemented to receive a digital television broadcast signal, and system 1600 may include a set-top box housing or a portable housing, such as a mobile telephone housing. System 1600 may be implemented in a smartphone, or may be implemented as part of a wireless router.

Methods and systems disclosed herein may be implemented in hardware, software, firmware, and combinations thereof, including discrete and integrated circuit logic, application specific integrated circuit (ASIC) logic, and microcontrollers, and may be implemented as part of a domain-specific integrated circuit package, and/or a combination of integrated circuit packages. Software may include a computer readable medium encoded with a computer program including instructions to cause a processor to perform one or more functions in response thereto. The computer readable medium may include one or more non-transitory mediums. The processor may include a general purpose instruction processor, a controller, a microcontroller, and/or other instruction-based processor.

Methods and systems are disclosed herein with the aid of functional building blocks illustrating functions, features, and relationships thereof. At least some of the boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed.

Various computer program, method, apparatus, and system embodiments are described herein.

A. Computer Program Product Embodiment

An embodiment includes a non-transitory computer readable medium encoded with a computer program, including instructions to cause a processor to:

cause one or more lights to transmit multiple light packets each including a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies; and

vary transmit start-times of the multiple light packets relative to each other to permit a receiver to sample each light packet at a different phase of a fixed sample timeline that is asynchronous to the bit period and the frequency.

The computer readable medium may further include instructions to cause the processor to:

cause a light imager configured to receive the transmitted multiple light packets to sample the received multiple light packets based on the asynchronous sample timeline, to produce samples of each received PSF and corresponding message that coincide in time with a corresponding one of the different phases;

detect the HRF of the SFD in each of the sampled light packets;

demodulate the PSF of the SFD from each of the sampled light packets;

demodulate the message comprising a series of bits, corresponding to each demodulated PSF, from each of the sampled light packets; and

declare the demodulated message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.

The instructions may further include instruction to cause the processor to, if at least two of the demodulated messages are declared valid, then perform forward error correction on the at least two valid demodulated messages to construct a corrected demodulated message.

The HRF may be intensity modulated at a frequency that is at least an order of magnitude greater than each of the two frequencies.

Each of the data bits may be represented as light that is intensity modulated over a bit period at the one of two frequencies indicative of the bit.

The PSF may be represented as light that is intensity modulated over at least one bit period at the predetermined one of the two frequencies.

The sample timeline may include consecutive sample times spaced apart so as to produce at least two samples per bit period.

The instructions to cause the light imager to sample may further include instructions to cause the light imager to sample the received light packets for a sample integration time to produce each of the samples, and adjacent ones of the different phases may be separated by at least the integration time.

In the computer readable medium:

the instruction to cause one or more lights of a transmitter to transmit may further include instructions to cause the processor to cause multiple, spatially separated lights each to transmit a respective one of the multiple light packets so that all of the light packets partially overlap in time; and

the instructions to cause the processor to vary may further include instructions to cause the processor to vary the start-times of the multiple light packets and, as a result, the start-times of their respective PSFs, so that the PSFs have successive start-times to establish the different phases as successive phases.

The instruction to cause one or more lights of a transmitter to transmit may further include instructions to cause the processor to cause a light to transmit the multiple light packets as successive contiguous light packets.

The instructions to cause the processor to vary may further include instructions to cause the processor to vary the time durations of the successive delimiters with respect to each other to establish the different phases.

The instructions to cause the processor to vary may further include instructions to cause the processor to vary the time duration of the HRF in the successive delimiters.

The different phases may include a first phase followed by remaining successive phases, and phase differences between the first phase and each of the remaining successive phases increase monotonically.

B. Apparatus Embodiment

An apparatus embodiment comprises:

a light transmitter including:

one or more lights to transmit multiple light packets each including a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies; and

a controller to vary transmit start-times of the multiple light packets relative to each other to permit a receiver to sample each light packet at a different phase of a fixed sample timeline that is asynchronous to the frequency.

The apparatus may further comprise a light receiver to receive the multiple transmitted light packets, the light receiver including:

a light imager to sample the multiple received light packets based on the asynchronous fixed sample timeline, to produce samples of each received light packet that coincide in time with a corresponding one of the different phases; and

processing modules to

detect the HRF of the SFD in each of the sampled light packets;

demodulate the PSF from each of the sampled light packets;

demodulate the message comprising a series of bits, corresponding to each demodulated PSF, from each of the sampled light packets; and

declare a demodulated message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.

The processing module may be further configured to, if at least two of the demodulated messages are declared valid, then perform forward error correction on the at least two valid demodulated data messages to construct a corrected demodulated data message.

The HRF may be intensity modulated at a frequency that is at least an order of magnitude greater than each of the two frequencies.

In the apparatus:

each of the data bits may be represented as light that is intensity modulated over a bit period at the one of the two frequencies indicative of the data bit;

the PSF may be represented as light that is intensity modulated over at least one bit period at the predetermined one of the two frequencies indicative of the PSF;

the sample timeline may include consecutive sample times spaced apart so as to produce at least two samples per bit period;

the light imager may be configured to sample the received light packets for a sample integration time to produce each of the samples; and

adjacent ones of the different phases may be separated by at least the integration time.

In the apparatus:

the one or more lights may include multiple, spatially separated lights each to transmit a respective one of the multiple light packets so that all of the light packets are spatially-separated and partially overlap in time; and

the controller may be configured to vary start-times of the multiple light packets and, as a result, the start-times of their respective PSFs, so that the PSFs have successive start-times to establish the different phases as successive phases.

In the apparatus:

the one or more lights may include a light to transmit the multiple light packets as successive contiguous light packets; and

the controller is configured to vary the time durations of the successive delimiters with respect to each other to establish the different phases.

The controller may be further configured to vary the time duration of the HRFs in the successive delimiters.

The different phases may include a first phase followed by remaining successive phases, and phase differences between the first phase and each of the remaining successive phases increase monotonically.

The apparatus may further comprise:

a communication system to communicate with a network;

a processor to interface between the communication system and a user interface system; and

a housing,

wherein the processor, the communication system, and the light transmitter are positioned within the housing.

C. Method Embodiment

A method embodiment comprises:

transmitting multiple light packets each including a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies; and

varying transmit start-times of the multiple light packets relative to each other to permit a receiver to sample each light packet at a different phase of a fixed sample timeline that is asynchronous to the bit period and the frequency.

The method may further comprise:

receiving the multiple transmitted light packets;

sampling the multiple received light packets based on the asynchronous fixed sample timeline, to produce samples of each received light packet that coincide in time with a corresponding one of the different phases;

detecting the HRF in each of sampled light packets;

demodulating the PSF from each of the sampled light packets;

demodulating the message comprising a series of bits, corresponding to each demodulated PSF, from each of the sampled light packets; and

declaring a demodulated message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.

The method may further comprise, if at least two of the demodulated data messages are declared valid, then performing forward error correction on the at least two valid demodulated data messages to construct a corrected demodulated data message.

The HRF may be intensity modulated at a frequency that is at least an order of magnitude greater than each of the two frequencies.

In the method:

each of the data bits may be represented as light that is intensity modulated over a bit period at the one of the two frequencies indicative of the bit;

the PSF may be represented as light that is intensity modulated over at least one bit period at the predetermined one of the two frequencies indicative of the PSF;

the sample timeline may include consecutive sample times spaced apart so as to produce at least two samples per bit period;

the sampling may include sampling the received light packets for a sample integration time to produce each of the samples; and

adjacent ones of the different phases may be separated by at least the integration time.

The transmitting may include transmitting the multiple light packets so that all of the light packets are spatially-separated and partially overlap in time

The varying may include varying the start-times of the multiple light packets and, as a result, the start-times of their respective PSFs, so that the PSFs have successive start-times to establish the different phases as successive phases.

The transmitting may include transmitting the multiple light packets as successive contiguous light packets.

The varying may include varying the time durations of the successive delimiters with respect to each other to establish the different phases.

The varying of the time durations of the successive delimiters includes varying the time durations of the HRFs.

The different phases may include a first phase followed by remaining successive phases, and phase differences between the first phase and each of the remaining successive phases increase monotonically. 

What is claimed is:
 1. A non-transitory computer readable medium encoded with a computer program, including instructions to cause a processor to: cause one or more lights to transmit multiple light packets each including a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies; and vary transmit start-times of the multiple light packets relative to each other to permit a receiver to sample each light packet at a different phase of a fixed sample timeline that is asynchronous to the bit period and the frequency.
 2. The computer readable medium of claim 1, further including instructions to cause the processor to: cause a light imager configured to receive the transmitted multiple light packets to sample the received multiple light packets based on the asynchronous sample timeline, to produce samples of each received PSF and corresponding message that coincide in time with a corresponding one of the different phases; detect the HRF of the SFD in each of the sampled light packets; demodulate the PSF of the SFD from each of the sampled light packets; demodulate the message comprising a series of bits, corresponding to each demodulated PSF, from each of the sampled light packets; and declare the demodulated message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.
 3. The computer readable medium of claim 2, wherein the instructions further include instruction to cause the processor to, if at least two of the demodulated messages are declared valid, then perform forward error correction on the at least two valid demodulated messages to construct a corrected demodulated message.
 4. The computer readable medium of claim 2, wherein the HRF is intensity modulated at a frequency that is at least an order of magnitude greater than each of the two frequencies.
 5. The computer readable medium of claim 4, wherein: each of the data bits is represented as light that is intensity modulated over a bit period at the one of two frequencies indicative of the bit; the PSF is represented as light that is intensity modulated over at least one bit period at the predetermined one of the two frequencies; the sample timeline includes consecutive sample times spaced apart so as to produce at least two samples per bit period; the instructions to cause the light imager to sample further include instructions to cause the light imager to sample the received light packets for a sample integration time to produce each of the samples; and adjacent ones of the different phases are separated by at least the integration time.
 6. The computer readable medium of claim 1, wherein: the instruction to cause one or more lights of a transmitter to transmit further include instructions to cause the processor to cause multiple, spatially separated lights each to transmit a respective one of the multiple light packets so that all of the light packets partially overlap in time; and the instructions to cause the processor to vary further include instructions to cause the processor to vary the start-times of the multiple light packets and, as a result, the start-times of their respective PSFs, so that the PSFs have successive start-times to establish the different phases as successive phases.
 7. The computer readable medium of claim 1, wherein: the instruction to cause one or more lights of a transmitter to transmit further include instructions to cause the processor to cause a light to transmit the multiple light packets as successive contiguous light packets; and the instructions to cause the processor to vary further include instructions to cause the processor to vary the time durations of the successive delimiters with respect to each other to establish the different phases.
 8. The computer readable medium of claim 7, wherein the instructions to cause the processor to vary further include instructions to cause the processor to vary the time duration of the HRF in the successive delimiters.
 9. The computer readable medium of claim 1, wherein the different phases include a first phase followed by remaining successive phases, and phase differences between the first phase and each of the remaining successive phases increase monotonically.
 10. An apparatus, comprising: a light transmitter including: one or more lights to transmit multiple light packets each including a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies; and a controller to vary transmit start-times of the multiple light packets relative to each other to permit a receiver to sample each light packet at a different phase of a fixed sample timeline that is asynchronous to the frequency.
 11. The apparatus of claim 10, further comprising a light receiver to receive the multiple transmitted light packets, the light receiver including: a light imager to sample the multiple received light packets based on the asynchronous fixed sample timeline, to produce samples of each received light packet that coincide in time with a corresponding one of the different phases; and processing modules to detect the HRF of the SFD in each of the sampled light packets; demodulate the PSF from each of the sampled light packets; demodulate the message comprising a series of bits, corresponding to each demodulated PSF, from each of the sampled light packets; and declare a demodulated message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.
 12. The apparatus of claim 11, wherein the processing modules are further configured to, if at least two of the demodulated messages are declared valid, then perform forward error correction on the at least two valid demodulated data messages to construct a corrected demodulated data message.
 13. The apparatus of claim 11, wherein the HRF is intensity modulated at a frequency that is at least an order of magnitude greater than each of the two frequencies.
 14. The apparatus of claim 13, wherein: each of the data bits is represented as light that is intensity modulated over a bit period at the one of the two frequencies indicative of the data bit; the PSF is represented as light that is intensity modulated over at least one bit period at the predetermined one of the two frequencies indicative of the PSF; the sample timeline includes consecutive sample times spaced apart so as to produce at least two samples per bit period; the light imager is configured to sample the received light packets for a sample integration time to produce each of the samples; and adjacent ones of the different phases are separated by at least the integration time.
 15. The apparatus of claim 10, wherein: the one or more lights include multiple, spatially separated lights each to transmit a respective one of the multiple light packets so that all of the light packets are spatially-separated and partially overlap in time; and the controller is configured to vary start-times of the multiple light packets and, as a result, the start-times of their respective PSFs, so that the PSFs have successive start-times to establish the different phases as successive phases.
 16. The apparatus of claim 10, wherein: the one or more lights includes a light to transmit the multiple light packets as successive contiguous light packets; and the controller is configured to vary the time durations of the successive delimiters with respect to each other to establish the different phases.
 17. The apparatus of claim 16, wherein the controller is further configured to vary the time duration of the HRFs in the successive delimiters.
 18. The apparatus of claim 10, wherein the different phases include a first phase followed by remaining successive phases, and phase differences between the first phase and each of the remaining successive phases increase monotonically.
 19. The apparatus of claim 11, further comprising: a communication system to communicate with a network; a processor to interface between the communication system and a user interface system; and a housing, wherein the processor, the communication system, and the light receiver are positioned within the housing.
 20. An apparatus, comprising: a light imager to: receive multiple light packets each including a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies, the multiple light packets having different start-times relative to each other; and sample the multiple received light packets based on a fixed sample timeline that is asynchronous to the frequency, to produce samples of each received light packet that coincide in time with a corresponding one of the different phases; and processing modules to: detect the HRF in each of the sampled light packets; demodulate the PSF from each of the sampled light packets; demodulate the message comprising a series of bits, corresponding to each demodulated PSF, from each of the sampled light packets; and declare a demodulated message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.
 21. The apparatus of claim 20, wherein the processing modules are further configured to, if at least two of the demodulated messages are declared valid, then perform forward error correction on the at least two valid demodulated data messages to construct a corrected demodulated data message.
 22. An method, comprising: transmitting multiple light packets each including a sequence of fields of light modulated to indicate (i) a delimiter, including a high rate frequency field (HRF) and a predetermined phase synchronization field (PSF), and (ii) a same message comprising a series of data bits each intensity modulated at a selected one of two frequencies to indicate the data bit, wherein the PSF is intensity modulated at a predetermined one of the two frequencies; and varying transmit start-times of the multiple light packets relative to each other to permit a receiver to sample each light packet at a different phase of a fixed sample timeline that is asynchronous to the bit period and the frequency.
 23. The method of claim 22, further comprising: receiving the multiple transmitted light packets; sampling the multiple received light packets based on the asynchronous fixed sample timeline, to produce samples of each received light packet that coincide in time with a corresponding one of the different phases; detecting the HRF in each of sampled light packets; demodulating the PSF from each of the sampled light packets; demodulating the message comprising a series of bits, corresponding to each demodulated PSF, from each of the sampled light packets; and declaring a demodulated message as a valid demodulated message if the corresponding demodulated PSF matches the predetermined PSF.
 24. The method of claim 23, further comprising, if at least two of the demodulated data messages are declared valid, then perform forward error correction on the at least two valid demodulated data messages to construct a corrected demodulated data message.
 25. The method of claim 23, wherein the HRF is intensity modulated at a frequency that is at least an order of magnitude greater than each of the two frequencies.
 26. The method of claim 25, wherein: each of the data bits is represented as light that is intensity modulated over a bit period at the one of the two frequencies indicative of the bit; the PSF is represented as light that is intensity modulated over at least one bit period at the predetermined one of the two frequencies indicative of the PSF; the sample timeline includes consecutive sample times spaced apart so as to produce at least two samples per bit period; the sampling includes sampling the received light packets for a sample integration time to produce each of the samples; and adjacent ones of the different phases are separated by at least the integration time.
 27. The method of claim 22, wherein: the transmitting includes transmitting the multiple light packets so that all of the light packets are spatially-separated and partially overlap in time; and the varying includes varying the start-times of the multiple light packets and, as a result, the start-times of their respective PSFs, so that the PSFs have successive start-times to establish the different phases as successive phases.
 28. The method of claim 22, wherein: the transmitting includes transmitting the multiple light packets as successive contiguous light packets; and the varying includes varying the time durations of the successive delimiters with respect to each other to establish the different phases.
 29. The method of claim 28, wherein the varying of the time durations of the successive delimiters includes varying the time durations of the HRFs.
 30. The method of claim 22, wherein the different phases include a first phase followed by remaining successive phases, and phase differences between the first phase and each of the remaining successive phases increase monotonically. 